Detection of grounded anodes

ABSTRACT

In a multiple anode alumina reduction cell, gas bubbles are formed on the bottom of each anode block during the reduction process and rise to the surface of the molten cryolite. The formation and subsequent release of bubbles causes variations in current flowing through the anode block. When an anode block is grounded, the rate at which gas bubbles are produced is significantly less, for a given anode block current, than when the anode block is not grounded. Apparatus is provided to sense current in the anode block and, from analysis of the variation in that current determines the presence of a ground or electronic path. The measured current varies as a result of factors other than the gas bubbles so that the measurement signal is filtered to obtain a signal whose frequency of amplitude variation is in the range of interest. A comparator compares the magnitude of the frequency signal with a reference to eliminate variations due to noise, and the comparator produces digital pulses that are fed to a first counter. The measurement signal is also fed to a voltageto-frequency converter and the output of the converter drives a second counter. The counts in the two counters are compared and the resulting signal indicates whether or not a grounded anode condition exists. A data processor automatically controls plural pot lines each having plural cells so that the measurements are taken automatically. An addressable multiplexer is provided for each cell so that only one ground detector is required for each pot line.

United States Patent 1191 Richards et al.

[ Apr. 1, 1975 DETECTION OF GROUNDED ANODES [73] Assignee: ReynoldsMetals Company,

Richmond,.Va.

22 Filed: -Sept. 17, 1973 21 Appl. No.: 398,286

[52] US. Cl. .l 204/67, 204/228 Primary Examiner-John H. Mack AssistantE.\'aminerD. R. Valentine Attorney, Agent, or FirnI-Glenn, Palmer, Lyne& Gibbs [57] ABSTRACT In a multiple anode alumina reduction cell, gasbubbles are formed on the bottom of each anode block during thereduction process and rise to the surface of the molten cryolite. Theformation and subsequent release of bubbles causes variations in currentflowing through the anode block. When an anode block is grounded, therate at which gas bubbles are produced is significantly less, for agiven anode block current, than when the anode block is not grounded.Apparatus is provided to sense current in the anode block .and, fromanalysis of the variation in that current determines the presence of aground or electronic path. The measured current varies as a result offactors other than the gas bubbles so that the measurement signal isfiltered to obtain a signal whose frequency of amplitude variation is inthe range of interest. A comparator compares the magnitude of thefrequency signal with a reference to eliminate variations due to noise,and the comparator produces digital pulses that are fed to a firstcounter. The measurement signal is also fed to a voltage-to-frequencyconverter and the output of the converter drives a second counter. Thecounts in the two counters are compared and the resulting signalindicates whether or not a grounded anode condition exists. A dataprocessor automatically controls plural pot lines each having pluralcells so that the measurements are taken automatically. An addressablemultiplexer is provided for each cell so that only one ground detectoris required for each pot line.

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SHEET 8 BF 8 PERIOD BETWEEN FLUCTUATIONS (SECONDS) FIG.7

DETECTION OF GROUNDED ANODES BACKGROUND OF THE INVENTION This inventionrelates to a method of, and apparatus for, detecting conditions ofsignificant anode maladjustment of interelectrode distance in an aluminareduction cell. More particularly, the present invention provides amethod of, and apparatus for, automatically detecting a grounded anode,or maladjustment of an anode in a multiple anode alumina reduction cell.

In U.S. Pat. No. 3,345,273 issued to Robert V. Brown, there is discloseda method and apparatus for indicating anode position. The methoddisclosed in that patent is based on the discovery that the voltage waveform across a multiple block anode alumina reduction cell comprises asubstantial direct current wave, with a random low frequency alternatingcurrent wave of a fluctuating amplitude superimposed thereon. Theamplitude of the superimposed wave varies as a function of the degree ofoverloading of one or more carbon anodes in the cell. When one or moreof the anodes is closer to the molten aluminum layer than the majorityof the anodes in the cell, the amplitude variations of the superimposedalternating current are of magnitudes sufficient to be detected andindicated on a suitable meter. When all of the anodes are in properrelationship to the molten aluminum layer, the alternating currentsignal is comparatively small and the reading on the meter isproportionately lower. This variation in amplitude of the alternatingcurrent signal may be used to accurately indicate when the anodes ofacell should be adjusted. However, this method merely provides anindication that at least one of the individual anodes of a cell shouldbe adjusted, but provides no indication as to which particular anode oranodes require the adjustment.

During the electro-chemical reduction of alumina, the carbon anodes ofthe reduction cell are consumed by a process that produces carbondioxide gas. This gas is released as bubbles from the lower surfaces ofthe carbon anodes. The present invention is based on the discovery thatfor a given electrolyte composition and hydrostatic pressure (anodeemersion) the bubbles progress in size from the minimum for nucleationto a fairly narrow range of sizes (volume) at which time the bubblesovercome hydrostatic pressure and there is a rapid movement of thebubble from underneath the surface of the anode toward the atmosphere atthe top of the cell. The number of bubbles is thus proportional to therate of production of aluminum or the anode current density on the lowersurface of the anode.

The growth and release of bubbles causes increases and decreases in thecontact area between the electrolyte and the anode block, hence thecurrent through each anode block fluctuates between a maxima and aminima at a rate dependent upon the rate of gas evolution at the anodeblock. It has been found that when an individual carbon anode isgrounded, there is a significant reduction in the rate in which gas isproduced at that anode block. Consequently, fewer gas bubbles are formedand released and there are fewer fluctuations of the anode current. Thebasic reason for this is that with the carbon anode grounded a portionof the total current flowing through the individual anode conductivelyflows directly into the molten aluminum and thus does not aid in thereduction process. This wasted current represents a large economic lossand, as explained ir the aforementioned patent, may lead to anadditionaj current flow through the individual anode which overheats themetal stub holding the carbon block, thus melting the stub and allowingthe carbon block to be disconnected from the anode rod. Even thoughconditions may be such that a grounded anode with an electronic path inparallel with the electrolytic path may carry more than itsproportionate share of the line current, this is not always so. Agrounded anode may actu ally carry less than the average current.Therefore measurement of the magnitude of anode current alone cannotprovide the basis for determining a grounder anode condition.

SUMMARY OF THE INVENTION An object of the present invention is toprovide z novel method and apparatus for detecting a grounder ormaladjusted anode in a reduction cell.

An object of the present invention is to provide 2 method of detecting agrounded or maladjusted anode comprising the steps of: measuring thetotal curren' through the anode over an interval of time and generatinga reference value corresponding to the number 0: current fluctuationsthat should result from the forma tion and release of gas bubbles at theanode during the interval the current is measured; deriving from themeasured current a second value corresponding approximately to thenumber of current fluctuations tha actually occur during the measuringinterval, and com paring the reference value to the second value.

An object of the present invention is to provide 2 method as set forthin the preceding paragrapl wherein a fluctuating voltage proportional tothe tota current is produced, the voltage converted to a se quence ofpulses, and the pulses counted to produce the reference value; andwherein the voltage is filterec to eliminate fluctuations at frequenciesoutside the range at which fluctuations occur as a result of the formation and release of the gas bubbles to thereby pro duce a secondsequence of pulses, and said second se .quence of pulses is counted oversaid time interval tr derive the second value, a grounded anode beingpres ent if the comparison step indicates that the reference value isgreater than the second value.

A further object of the invention is to provide appa ratus forautomatically performing the steps of thl method set forth above.

In accordance with one aspect of the invention, appa ratus for detectinga grounded or maladjusted anodl comprises means for producing afluctuating voltag corresponding to total current flow through the anodevoltage to frequency converter means for convertin; the voltage to afirst sequence of pulses having a fre quency proportional to the rate atwhich gas bubble should be formed and released at the anode when thanodeis not grounded or maladjusted and the mea sured current flowstherethrough, pulse shaper mean responsive to the voltage for producinga second se quence of pulses wherein each pulse corresponds to thformation and release of a gas bubble at the anode counter means forcounting the sequences of pulses and comparison means for comparing thecounts in th two counters and producing a signal indicating whethe theanode is grounded or not properly adjusted.

Yet another object of the invention is to provide a: apparatus asdescribed above wherein the pulse shape includes filter means forfiltering from said voltage fluctuations occurring at a rate outside therange of rates at which the gas bubbles might be produced, rectifiermeans responsive to the filtered voltage, and comparator meansresponsive to the filter means and the rectifier means for producing thepulses in the second sequence of pulses.

Other objects of the invention and its mode of operation will becomeapparent upon consideration of the following description and theaccompanying drawing.

BRIEF DESCRIPTION OF DRAWING FIG. 1 is a side view, partly in section,showing a prior art multiple anode alumina reduction cell;

FIG. 2 is a block diagram of a circuit for detecting a grounded ormaladjusted anode;

FIG. 3 is a schematic wiring diagram of a pulse shaper such as that usedin FIG. 2;

FIG. 4 is a block diagram of a data processor and multiplexer system forcontrolling a plurality of pot lines;

FIG. 5 is a logic diagram of the circuits employed in the section boxassociated with one pot line;

FIGS. 6A-6C are logic diagrams showing the circuits of a multiplexer forcontrolling one reduction cell; and,

FIG. 7 is a graph showing the relationship between anode current andfluctuations in anode voltage for a typical adjusted cell.

DESCRIPTION OF A PREFERRED EMBODIMENT THE REDUCTION CELL For purposes ofillustrating the invention, FIG. 1 shows a prior art alumina reductioncell of a type known by various names such as prebake, Niagara, etc.However, it will become clear from the following description that thepresent invention is not limited in use to cells of the type illustratedin FIG. 1. As shown in FIG. 1, the cell includes a plurality, i.e., Ncarbon block anodes 11 each connected to a copper anode rod or stem 12by means of a metal stub 14 cast in the carbon anode block. Each rod 12is clamped to an anode bus 16 by means of a hand-operated clamp 18. Theclamps permit an operator, with the aid of a conventional hand jackgenerally used in the industry, to raise or lower one anode block 1 1relative to the others. A motordriven bridge jack 19, attached to a cellframe 21, drives the anode bus 12 so that all of the anode blocks mayberaised or lowered in unison.

A low voltage, high current source (not shown) has its positive sideconnected to the anode bus 16 and its negative side connected to acathode bus 23. The cathode bus is connected by means of currentcollectors 24 to a carbon cathode 26. During operation of the cell eachcarbon block 11 is maintained with its lower surface in contact with alayer of molten cryolite 28. As the reduction process takes place, alayer of molten aluminum 30 forms adjacent the cathode 26 while oxygencombines with the carbon blocks 11 to form gas bubbles (oxides ofcarbon) at the lower faces of the carbon blocks. FIG. 1 shows one gasbubble 32 as it is being formed at the lower face of the right-mostcarbon block 11 Depending upon the hydrostatic pressure at the lowerface of each carbon block, the bubbles build up to a certain size (withlimits) before they escape around the carbon blocks to the upper surfaceof the cell. A gas bubble 33 is shown as it is released from the lowersurface of a carbon block and begins movement into the atmosphere abovethe cell. In the following description a carbon block 11 is referred tosimply as an anode.

Each anode stem 12 has an anode current measuring means connectedthereto for deriving a voltage proportional to the current flowingthrough the stem. This current measuring means comprises two electricalleads 38 and 40 connected at separate points along the stem. Because ofthe electrical resistance of the anode stem to current flowingtherethrough, a voltage differential, sometimes referred to as the stemvoltage, exists between the two points on the stem and this voltageappears on the leads 38 and 40. This method of measuring anode currentflow is well known in the art.

It has been found that as long as a particular anode 11 is properlyadjusted, then for a given current through anode, gas bubbles 32 areformed and released at a fairly constant rate. As each bubble increasesin size it decreases the area of contact between the lower surface ofthe anode 11 and the layer of molten cryolite 28. This in effect causesa gradual increase in the anode resistance and results in acorresponding decrease in current through the anode. As each bubble isreleased, the area of contact between the anode and the cryolite againincreases with the result that the current through the anode againincreases. Thus, during normal cell operation the stem voltage appearingacross leads 38 and 40 is a DC voltage that fluctuates slowly in agenerally sinusoidal fashion at a frequency corresponding to thefrequency at which bubbles are formed and released at the anode.

When an anode 11 is groundedto the layer of molten aluminum 30, or ismaladjusted to such an extent that it is incipiently grounded, aconductivecurrent path is established from anode bus 16, through anodestem 12, anode 11, aluminum layer 30, and current conductors 24, to thecathode bus 23. This conductive current is wasted and contributesnothing to the reduction process. Since the conductive current does notcontribute to the reduction process, fewer gas bubbles 32 are formed atthe anode 11 for a given current through the anode. As a result, for agiven anode current, the stem voltage across leads 38 and 40 fluctuatesat a lower frequency when the anode is grounded or maladjusted than whenit is in normal operating condition.

From the above description it is seen that the presence of a groundedanode, an incipiently grounded anode, or an anode in need of verticaladjustment with respect to the liquid cathode, may be detected by amethod including the steps of determining the frequency of the voltagefluctuations appearing across leads 38 and 40 and comparing thisfrequency with what the normal frequency of the fluctuations should befor the amount of current flowing through the anode. When the anode isgrounded or is vertically maladjusted so that there is an electronicconduction path from the anode to the liquid cathode, the frequency issignificantly less for a given current flow than for normal orungrounded anode operation with the same current flow.

Since the frequency of fluctuations for a given current flow through ananode is dependent on the particular cell, it is necessary to establishthe normal relationship between electrolytic current flow and thefrequency of the fluctuations resulting from gas bubble release. This isaccomplished by properly adjusting the anodes of a cell, varying thecurrent through the anodes, and plotting a graph of total (individual)anode currents against the frequency or period between fluctuations.

FIG. 7 is a graph of total anode current versus the period betweenfluctuations due to gas bubble release for an anode of a typical cell.The graph is obtained by applying anode stem voltages, one at a time toa conventional X-Y plotter and recording the fluctuations as a functionof time. By visual analysis, i.e., by counting the peaks of thegenerally sinusoidal trace over an interval of time, the period betweeenfluctuations for that particular anode current is determined. This fixesone point on the graph of FIG. 7. The anode current is then changed andthe process repeated to determine another point on the graph. To obtainthe graph of FIG. 7, the anode current was varied in steps between about13,750 amps and 5,500 amps and a point on the graph establishd for eachstep. The data of the graph of FIG. 7 was obtained over a five dayperiod with measurements being made each day on from two to 12 anodes inan 18 anode cell. However, the measurements need not be made over such along interval of time.

It will be understood that because of various factors occurring duringthe measurement, and the error inherent in measurements of the typedescribed above, all of the computed points on the graph of FIG. 7 donot fall on a straight line. Thus, the line C represents the curve ofbest fit for the points plotted. All of the measurements resulted inplotting points falling within the 95 percent confidence limitsrepresented by the dashed lines of FIG. 7. With the method describedabove, it was found that the period for a given anode current wasreproducible within 10.03 seconds.-

The normal relationship between electrolytic current flowing through ananode, and the period between fluctions is represented by the equationwhere I is the electrolytic current flowing through the cell, Y is theintercept obtained by extrapolating the curve C of FIG. 7, dy/dx is theslope of the curve C, and T is the time in seconds between fluctuations.Having once established this normal relationship for a properly adjustedcell, measurements may later be made while the cell is in operation todetermine which, if any, of its anodes are grounded or improperlyadjusted.

TYPICAL EXAMPLES The following examples illustrate how theabovedescribed method may be used for the analysis of anode adjustment.All examples are for a reduction furnace having 18 prebaked anodes.Example I. A manual analysis was made by recording anode stem voltagesproportional to current for an interval of 30 to 40 seconds. Fromanalysis of the recordings for all 18 anodes it was established, asexplained above with respect to FIG. 7, that the period betweensuccessively released gas bubbles should be between 0.4 and 0.6 secondsfor an anode carrying 12,000 amps. One anode carrying 12,000 amps showeda periodicity in the sinusoidal waveform of between 1.10 and 1.25seconds. Partial electronic conduction was suspected. When the anode wasremoved for inspection it was found to have a projection.

The grounded anode was raised two inches. With the anode current at7,300 amps the period of bubble release was redetermined and found to be0.951015 sec.

From the normal relationship between current and period established bymeasurement of all the anode currents, the period should have been0.86i12 sec. This indicated that the anode was no longer grounded. Aphysical check of the anode confirmed that the projection was no longercontacting the metal pad. Example II. In this example, the anode stemvoltages were sensed for about 30 seconds each applied to the filtercircuit of FIG. 3, described below, and the filtered signals applied tothe X-Y plotter. Determination of the period between fluctuations wasmade manually by counting the number of fluctuations per unit of time onthe recorded trace.

The normal relationship was determined to be 1,; 14,600 6150T.

Upon subsequent measurement it was found that one anode carrying 8,800amps was releasing gas bubbles every 1.40 seconds. This was threestandard deviations from the normal relationship and the anode wasdiagnosed as grounded. Upon removal for inspector it was found to have aprojection into the metal pad. Example 111. In this example, themeasurements were made as in Example 11, but with the differentialamplifier 208 (FIG. 2) connected to the input of the filter circuit.

The normal relationship was determined from readings on 15 anodes to be1,; 16,000 7,700T.

Upon subsequent measurement, one anode carrying 15,200 amps wasreleasing a gas bubble about every 0.77 second, and one anode carrying10,700 amps was releasing a gas bubble every 1 second. These points were3 and 7 standard deviations, respectively, outside the zone ofreasonable error in the normal relationship. The anodes were predictedto be grounded. When raised for inspection, the first anode was found tohave a white hot projection 6 inches in diameter, and the latter anodewas found to have a projection 1.5 inches in diameter. Example IV. Bythe manual method of Example 1, two anodes were determined to beproperly adjusted and carrying 1 1,300 and 8,400 amps. The fluctuationswere determined to be occurring at intervals of 0.66 and 1.02 seconds,respectively.

Another measurement was then made at the same anode current levels usingthe amplifier 208, filtershaper 203, and counter 213 of FIG. 2. Thismeasurement determined the period between fluctuations for the twoanodes to be 0.72 and 1.0 seconds, respectively, thus indicating theoperativeness of the electronic apparatus for making the measurements.

GROUND DETECTOR CIRCUITS While referred to as ground detector circuits,it will be evident that the circuits subsequently described may beemployed to detect a grounded anode, an incipientlygrounded anode, or,in general an electronic conductive path between an anode and the liquidcathode in a reduction cell. In a Niagara aluminum reduction cell withan anode in need of vertical adjustment, the measured rate of stemvoltage leads will vary 0.8 to 1.2 standard deviations from the normalrelationship between anode current and voltage fluctuation.

FIG. 2 is a block diagram of a preferred embodiment of an apparatus fordetecting grounded anodes in accordance with the method described above.As subsequently explained, the stem voltage signals appearing acrossleads 38 and 40 are multiplexed so that they are applied one at a timeto the input leads 201 and 202 of FIG. 2. However, for purposes of thepresent description, assume that the leads 38 and 40, of FIG. 1 aredirectly connected to the leds 201 and 202 respectively. Thus, the stemvoltage representing current flow through the rightmost anode 11(FIG. 1) is applied over leads 201 and 202 to a differential amplifier208. The output of amplifier 208 is connected to the inputs of a pulseshaper 203 and a voltage-to-frequency converter 205.

Pulse shaper 203 is subsequently described in detail but, generallyspeaking, it filters out noise from the incoming signal and produces anoutput lead 207 a sequence of pulses with each pulse corresponding tothe formation and release of one gas bubble at the rightmost anode 11The output pulses from pulse shaper 203 are applied over the lead 207 toa counter 213 which accumulates a count representing the actual numberof bubbles released during a given interval.

The voltage-to-frequency converter 205 is designed such that over agiven interval of time it produces on an output lead 209 a number ofpulses corresponding to the number of gas bubbles which should be formedand released at an anode 11, if the anode is not grounded. Thus, theconversion ratio may vary depending upon the type of cell beingmonitored, and should be adjusted accordingly when the ground detectorapparatus is first set up. The pulses on lead 209 are applied to acounter 211 to provide a digital standard of bubble count against whichthe actual bubble count may be compared.

The circuit of FIG. 2 operates as follows. A reset pulse is applied overa lead 215 to reset both the counters 211 and 213. After termination ofthe reset pulse a gating pulse is applied to both of the counters over alead 217. This pulse may last for a considerable length of time, say 30seconds, and during this 30 second interval conditions both counters 211and 213 to receive the pulses applied to them over leads 209 and 207,respectively. At the endof the 30 second interval the gate pulse on lead217 is terminated. At this time the counter 211 contains a countcorresponding to the number of bubbles which should have been releasedfrom underneath the anode 11 during the 30 second interval, and thecpunter 213 contains a count of the number of bubbles actually releasedduring that interval. The outputs from counters 211 and 2.13 are appliedto a digital comparator 219 which compares the two counts and determineswhether they are equal or one is greater than the other. If the count incounter 213 is less than the count in counter 211, comparator 219produces a signal on lead 221 to condition an output level selector 223.If the count in counter 211 is equal to or greater than that in counter213 then the comparator 219 produces a signal on lead 225 or 227,respectively, to condition the output level selector.

The output level selector 223 comprises a conventional gating means forgatingonto an output lead 229 one of three voltage levels, 5V, V, or +V,depending upon whether the selector is conditioned by a signal on lead221, 225 or 227, respectively.

In the simplest form of the invention, the voltage on lead 229 might beused to visually or audibly signal to an operator the grounded conditionof the anode. However, as subsequently explained, the output voltagelevels on lead 229 are fed to a data processor which controls aplurality of groups of cells each having a plurality of anodes 1 l, andthe data processor uses the signals to monitor and control variousoperations associated below the frequency of interest may result fromelectri-,

cal motors and other electrical apparatus found in the vicinity of thecell.

The pulse shaper comprises integrated circuits 301 through 310. Circuits301 through 309 are microoperational amplifiers, for example FairchildType 741C, whereas circuit 310 may be a type 351K analog comparator suchas that commercially available from Analog Devices, Inc. For the sake ofclarity, the bias voltages and external connections for amplifiers 302through 309 are not shown but it should be understood that they are thesame as those shown for amplifier 301.

The voltage signal representing anode current is applied over lead 204to amplifier 301 which functions merely as a scaling amplifier. Theoutput of amplifier 301 is applied to a notch filter means comprisingamplifiers 302, 303, 304 and summing junction 312. More specifically,the output of amplifier 301 is applied to amplifier 302 through a filtercircuit, generally indicated at 314, so that that the output ofamplifier 302 includes signals of all frequencies less than the maximumfrequency at which bubbles are produced and released; The output ofamplifier 302 is applied to the summing junction 312 through a resistor316.

The output of amplifier 301 is also connected through a filter circuit,generally designated 318, to the input of amplifier 303. The filtercircuit 318 is such that the output of amplifier 303 is a signalcontaining only frequencies less than the minimum frequency at whichbubbles are produced and released. The output signal from amplifier 303is inverted by amplifier 304 and applied to summing junction 312,v sothat the input to scaling amplifier 305 is a signal comprising pulses oramplitude variations occurring at frequencies within the range offrequencies at which bubbles are produced and released. These pulses areamplified by amplifier 309 and applied to one input of the comparator310. In some cases it is possible to dispense with scaling amplifier 309and apply the output of amplifier 305 directly to the comparator. P

The pulses appearing at the output amplifier 305 are in the mature ofhalf sine waves centered about a zero voltage level. The pulses are alsoapplied over a lead 322 to the amplifier 306 and the output of thisamplifier is connected through a pair of diodes 324 and 326 to theamplifier 307. Amplifiers 306 and 307 together with the diodes 324 and326 provide full wave rectification and amplification of the outputsignal from amplifier 305. The output of amplifier 307 is then appliedto amplifier 308 which functions as a slow filter or integrator. As aresult, the output of amplifier 308 is a DC signal equal to the averagevalue of the peaks of the pulses produced at the output of amplifier305. The output of amplifier 308 is applied to the second input ofcomparator 310 so that the comparator produces a digital output pulse onlead 207 only for those intervals of time during which the output pulsesfrom amplifier 305 exceed in magnitude the DC average of the pulses.This has the effect of eliminating noise pulses or pulses of smallmagnitude which might result from conditions other than the formationand release of gas bubbles at the anode. The shaped pulses on lead 207are then applied to counter 213 as previously described.

MULTIPLEXER SYSTEM As previously explained, ground detector circuitssuch as those shown in FIGS. 2 and 3 might be provided for each anode ineach cell in order to monitor operations of the cells and detectgrounded anodes. However, in a typical reduction plant there may be, forexample, three pot lines each comprising 30 pots or cells, with eachcell having 18 carbon anode blocks. This means that for the entiresystem I620 circuits like those shown in FIGS. 2 and 3 would berequired. In accordance with one aspect of the invention, multiplexingmeans are provided for selectively connecting the voltages read at theanode stems 12 to a ground detector so that one ground detector mayperform the detection function for all of the anodes in all of the cellsof one pot line. Thus, for the assumed system configuration only 3,rather than l620 ground detector circuits would be required. Theinvention in its application is not limited to a system having thespecific number of pot lines, cells per pot line, or anode blocks percell, as assumed, but may be used in a system having more or fewer ofany or all of these elements.

FIG. 4 is a block diagram of the multiplexer system. A digital dataprocessor 400 controls 3 pot lines (only two of which are shown) witheach pot line including 30 potsor alumina reduction cells 402. A cellmultiplexer 404 is provided for each cell and a single section box 406is provided for each pot line. Each section box includes one grounddetector 408, an isolator 410, and a calibrating power supply 412.

As subsequently explained in greater detail, data read from a cell 402is transferred over a bus 414 to its associated multiplexer 404 andcontrol signals from the data processor are passed through themultiplexer and the bus 414 to perform various control functions in thecell. All of the multiplexers for one pot line are connected in parallelto a data bus 416 and this data bus is connected to the section box 406.The cell multiplexers for a given pot line are also connected inparallel to a ground detector bus 418 and this bus is connected to theground detector 408 in the section box. The output from the grounddetector 408 is tied to the data bus 416 so that either the data on bus416 or the output of the ground detector 408 may pass through theisolator 410 and over a connecting data bus 420 to the computerinterface circuits 422. Each of the cell multiplexers 404 is alsoconnected in parallel to a control bus 424 which extends through thesection box 406 to the computer interface circuits 422. As subsequentlyexplained, certain leads within control bus 424 are also connected tocircuits within the section box 406.

The control bus 424 contains 14 pairs of leads. One pair of leads is fortransmitting one binary bit representing an interrupt signal from thecell multiplexers to the data processor. The remaining thirteen pairs ofleads are for transmitting a thirteen bit command or control wordcontaining address, function, and control signals from the dataprocessor to the multiplexers and section box. Within the bus 424 arefive pairs of address lines which enable the data processor to addresseither the section box 406 for a pot line or any one of the cellmultiplexers in the pot line. It should be noted that with only fivepairs of address lines it is possible to address only 31 addresses.Thus, five pairs of address lines permits the addressing of 30multiplexers and the section box for each pot line. The selection of thepot line is determined by the program of the data processor whichdetermines which of the interface circuits 422 will be enabled to passthe address signals. For example, if the data processor is working theportion' of the program dealing with pot line 1, and the binary address00001 is generated, this address would pass through interface circuits422 and select the multiplexer 404 for cell 1 in the first pot line. Onthe other hand, if the data processor is working that section on theprogram relating to the third pot line, and generates the address 00001,then this address would pass through interface circuit 422;, to selectthe multiplexer 404 for cell 61.

SECTION BOX The section boxes 496 are all identical and the details of atypical section box are shown in FIG. 5. In the lower portion of thisFigure the control bus 424 is shown as several busses depending upon thefunctions performed by the signals appearing on the leads. The bus 424aincludes two leads for carrying a Function Reset signal, the bus 4242includes ten leads for carrying the five digit binary code representingthe function to be performed, and the bus 424f includes two leads forcarrying an Interrupt signal. These leads merely pass through thesection box on their way between the multiplexers and the data processorand are not connected to any of the circuits in the section box.

The control bus also includes an address bus 424a having ten leads forcarrying signals representing a 5bit binary address, a bus 424b havingtwo leads for carrying an Enable signal, and a bus 4240 having two leadsfor carrying a Ground Detection Enable signal. At this point it shouldbe noted that in the present system a pair of leads is required totransmit a signal representing a binary bit. A binary 1 is representedby a high level voltage signal on one lead concurrently with a low levelvoltage on the second lead of the pair. A binary 0 is represented by alow level signal on the first lead of a pair concurrently with a highlevel signal on the second lead.

Each section box is assigned the address 31. Five differential receivers502 are connected to the five pairs of address lines in the control bus424a and when the address 31 appears on the control bus all fivedifferential receivers produce a low level output signal. Thedifferential receivers are operational amplifiers connected in acomparator configuration. The output of each differential receiver isconnected through an inverter 504 to an input of a NAND gate 506. Whenthe address 31 is present on the control bus NAND 506 produces a lowlevel output signal. that is inverted by an inverter 508 and applied toone input of two NAND gates 510 and 512. A differential receiver 514 hasits inputs connected to the pair of leads in the control bus 424b whichcarries the Enable signal. When the signal on these leads specifiesEnable the differential receiver 514 produces a low level output signalthat disables NAND 512. At the same time, the output from thedifferential receiver is inverted by an inverter 516 to condition thesecond input of NAND 510. The NAND gate produces an output signal toenergize a solid state relay 518. As used herein, a solid state relaymay be a transistor or any combination of transistors for performing aspecified switching function. For an easy understanding of the presentdescription, a solid state relay is treated as though it were anelectro-mechanical relay, and is illustrated as such in the drawings.

The relay 518 has a set of normally open relay contacts 518a connectedin series with a solid state relay 520 across the power supply lines 522and 524. When the contacts 518a close the relay 520 is energized to opennormally closed contacts 520a and 520b and close normally open contacts520C and 520d. The opening of contacts 520a and 520b disconnects thedata bus 416 and the output of ground detector 408 from the input toisolator 410 while the closing of contacts 5200 and 520d connects theoutput of calibrate power supply 412 to the input of the isolator.

The section box is provided with two power supplies. The logic powersupply 526 provides the power for operating the various logic circuitsin the section box. The calibrate power supply 412 is a highly regulatedpower supply which is used for checking the isolator 410. By applying acontrol word containing address 31 and an Enable bit to the section boxin the manner just explained, the calibrate power supply may beconnected to the isolator to apply a known voltage level to the input ofthe isolator. The output of the isolator is transmitted to the dataprocessor over the bus 420 and from the value of the voltage received atthe data processor it may determine if the isolator 410 is functioningproperly. When the control word on control bus 424 is terminated thecircuits for connecting the calibrate power supply to the isolatorreturn to normal.

The section box is also addressed to read out to the data processor theoutput from the ground detector 408. The command, Read Ground Detectorincludes the address 31 with no Enable bit. The address 31 again causesthe output of NAND 506 to condition one input of NAND gate 510 and 512.However, in the absence of an Enable bit the differential receiver 514produces a high level output signal. This signal conditions the secondinput of NAND 512 at the same time that the signal is inverted at 516 toblock NAND gate 510. NAND 512 produces a low level output signal toenergize a solid state relay 528. Relay 528 has a set of normally opencontacts 528a connected in series with a further solid state relay 530across the power supply lines 522 and 524. When relay 530 is energizedit opens normally closed contacts 530a and 530b and closes contacts 530Cand 530d. This disconnects the data bus 416 from isolator 410 andconnects the output of the ground detector 408 to the isolator so thatthe output from the ground detector may pass through the isolator andover bus 420 to the data processor.

As soon as the command Read Ground Detector is terminated on the controlbus, the output from NAND 506 disables NAND 512 and the circuits forreading out the output from the ground detector all return to normal.

As explained with reference to FIG. 2, the counters in the grounddetector must be reset at the beginning of the measuring interval andthen the inputs of the counters must be enabled over a period of time toenable the counters to accumulate a count. The circuits for generatingthe counter reset pulse and the counter gating pulse are shown in FIG.5. As subsequently explained, the command Determine Status enables thestem voltage drop at one anode of one cell on the pot line to be appliedover bus 418 to the ground detector 408 for that pot line. The commandDetermine Status comprises the Ground Detector Enable bit with anaddress specifying the multiplexer controlling the cell anode whosestatus is to be determined. Two leads in the control bus 424e carry thevoltage levels representing the Ground Detection Enable bit. This bit isapplied to a differential receiver 532 and the output of the receiver532 drops to a low level at the time a measuring interval is to begin.The output of differential receiver 532 triggers a 15 millisecond singleshot multivibrator 534 and a 30 second timer 536. For a period of ISmi]- liseconds the multivibrator 534 applies a signal over the lead 215to the ground detector to reset the counters in the ground detector.During this l5 millisecond interval the output of multivibrator 524 isinverted by an inverter 538 to block one input of a NAND gate 540. TheNAND gate 540 has a second input that is conditioned by the output ofthe 30 second timer 536 as soon as the timer is triggered. At the end ofthe 15 millisecond reset interval the output of amplifier 538 goes to ahigh level to condition NAND 540. The output of NAND 540 is applied overlead 217 to the gating inputs of the counters in the ground detector408. At the end of the 30 second interval the output of the timer 536drops to a low level thus blocking NAND 540 and terminating the gatingpulse.

MULTIPLEXER Addressing and Function Decoding All of the multiplexers areidentical and the circuits for a typical multiplexer are shown in FIGS.6a-6c. Referring to FIG. 6a, the leads of the function bus 4242 areconnected to five differential receivers 601 which respond to thecombination of voltage levels on the leads to produce the five binaryfunction signals F 1-F 5. The signal F5 is inverted by an inverter 602to produce the function signal F5. The function signals Fl-F4 areapplied to a first function decoder 603 (FIG. 6b) and to a secondfunction decoder 604, a portion of which is shown in FIG. 6b and aportion of which is shown in FIG. 60. The decoders are both 4-to-l6 bitdecoders and the function signals-F 1-F4 serve to energize the decodersto select one of the 16 possible outputs from each of the decoders. Thefunction signal F5 is applied to the decoder 604 whereas the functionsignal F5 is applied to the function decoder 603. Thus, if the functionsignal F5 is present, the signals F1 -F4 may energize decoder 604whereasif the signal F5 is present the signals may energize the decoder 603.However, since the function signals appearing on the function bus 424eare applied simultaneously to all of the multiplexers on the pot line itis necessary to limit the function decoding operation to only thosefunctions intended for the particular multiplexer addressed. This isaccomplished as follows.

In FIG. 6a, the signals on the address bus 424a are applied to fivedifferential receivers 605' which have their outputs connected to inputterminals of a manual plug board 606. Two NAND gates 607 and 608shavemultiple inputs that are also connected to output terminals of the plugboard 606. The output of NAND 608 is connected by way of a lead 609 toone input of NAND 607 so as to form an extended NAND gate, as is wellknown in the art. Manually inserted plug wires 610 are used toselectively connect the outputs of the differential receivers 605 to theinputs of NAND 607 and NAND 608. Each multiplexer on the pot line isassigned a different address and the plug wiring 610 is such that whenthe combination of address signals on address bus 424a corresponds tothe address of the multiplexer a low level output signal is produced atthe output of NAND 607. The output of NAND 607 is connected through aninverter 611 and over leads 612 and 613 to the gates 614 and 615 (FIG.6b). Thus, when the multiplexer is addressed one of the functiondecoders 603 or 604 may be energized to produce an output signal on oneof its 16 output leads. If the function signal F is present then thedecoder 604 will produce an output signal on one of its 16 output leads,the particular output lead energized being determined by.,thecombigation of signals Fl-F4. On the other hand, if the signal F5 ispresent then the decoder 603 will produce an output signal on one of its16 output leads, the particular output lead energized being determinedby the combination of signals F1-F4.

Each output from the decoders 603 and 604 controls one ofa series offlip-flops (FF) 616-624 and each flipflop controls a function. A furtherfunction flip-flop 625 is provided to control the connection of thevarious anode stem voltages to the data bus 416 or the ground detectorbus 418.

All of the flip-flops are reset at the time certain commands are to beperformed by the multiplexer. The commands will contain an Enable bitwhich appears on bus 424b. This bit conditions a differential receiver626 (FIG. 6a) which produces a low level output signal. This signal isinverted by an inverter 627 and applied to one input of a NAND gate 628.The address applied to differential receivers 605 causes the output ofNAND 607 to go to a low level. The output of NAND 607 is inverted at 611to condition the second input of NAND gate 628. The gate produces a lowlevel output signal that is inverted by inverter 629 before beingapplied to a NOR circuit 630. The NOR circuit produces a low leveloutput signal when any input is at a high level. The low level output ofNOR 630 is applied over a lead 631 to the reset inputs of the functioncontrol flip-flops 616 625 (FIGS. 6b and 6c). Immediately thereafter,one of the decoders produces an output signal, as previously described,to set one of the flip-flops 616-624.

The various functions performed by a multiplexer in response to variouscommands will now be explained. Read Stem Voltage: This command causesthe multiplexer to connect the voltage sensing leads 38 and 40 (FIG. 1)for one anode stem to the data bus 416 so that the voltage may be sensedby the data processor. The command includes the address of themultiplexer, the function code, and the Enable bit is a binary l. Thefunction code identifies the particular anode of the addressed cell thatis to have its anode stern voltage drop read onto the data bus. Thus,the function code may represent any number between one and eighteen,assuming the cell has 18 anodes 11. The address and Enable bits resetthe function fiip-flops 616-625 and energize the decoder 603 or 604 aspreviously described. Assume that the function is 00001 so that decoder603 produces an output signal on lead 632. This signal sets the functionflip-flop 616. Two solid state relays 634 and 636, having normally opencontacts 634a and 636a, respectively, are connected to the output ofFF616. The output of the flip-flop energizes the relays 634 and 636 toclose the contacts 634a and 636a. The contacts 634a and 636a have oneside connected to the leads 38, and 40, respectively, which are attachedto the anode stem of the anode block 11 shown in FIG. 1. Thus, when theflip-flop 616 is energized the voltage at this anode stem is appliedthrough the contacts 634a and 636a to a pair of leads 638 and 640.

From the leads 638 and 640 the measured stem voltage is applied to' thedata bus 416 through a pair of contacts 650a and 650b. These contactsare closed at this time because FF625 is reset. The high level outputsignal on output lead 641 from the flip-flop is applied to one input ofa NAND gate 642 (FIG. 6a). When the multiplexer is addressed and NANDgate 607 produces a low level output signal, this signal is inverted byinverter 611 and conditions the second input of NAND gate 642. Thus,when FF625 is reset the gate 642 produces a low level output signal thatis inverted by an inverter 644 and applied over a leads 646 to FIG. 6bwhere it energizes a solid state relay 648. The relay has a single setof normally open contacts 648a connected in series across the powersupply with a mercury relay 650. The mercury relay 650 has two sets ofnormally open contacts 650a and 65% which connect the leads 638 and 640to the data bus 416. The stem voltage drop at the anode stem 11 is thusapplied to the data bus from whence it may pass through the section boxto the data processor. When the address on bus 4240 is terminated, theoutput of NAND 642 goes to the high level. This releases relay 648 (FIG.6B) which in turn releases relay 650 and its associated contacts 650aand 65012. When these contacts are opened the stem voltage isdisconnected from data bus 416. However, the function flip-flop 616remains energized and will be reset only when the multiplexer is againaddressed with a command that includes the enable bit.

The commands for reading the stem voltage drops at stems 2 through 18differ from the command for reading the stem voltge drop at stem 1 onlyin the function code. Thus, if the function code were 00010 the voltagedrop at stem 2 would be connected to the data bus, etc. and if thefunction code were 18 then the voltage drop at stem 18 would beconnected to the data bus. Referring to FIG. 6b, this requires 18function flipflops like FF616, each flip-flop controlling twosolid staterelays corresponding to 634 and 636, the relays having contactscorresponding to contacts 634a and 636a. Sixteen of the flip-flops arecontrolled by decoder 603 but only the one flip-flop 616 is shown in thedrawing. Two of the flip-flops are controlled by decoder 604 and onlyone of these, i.e., FF617 is shown. Determine Status: This commandcomprises an address and a binary one bit on the ground detection enablebus 424c. No function code, or enable bit is required. However, thecommand must be preceded by a command Read Steam Voltage which reads thestem voltage drop at the anode whose grounded/ungrounded status is to bedetermined. The Read Stem Voltage command leaves the function flip-flop,such as FF616 for stem 1, set so that the stem voltage drop appearsacross leads 638 and 640. Subsequently, the Determine Status commandenergizes the differential receiver 652 (FIG. 6b) and the output of thereceiver is applied over a lead 654 to set FF625. At this time the lowlevel output signal from the flip-flop energizes solid state relay 655and the relay closes its contacts 655a.

The contacts 655a are connected in series with a mercury relay 656having two sets of normally open contacts 656a and 656b. When contacts6550 are closed relay 656 is energized to thus close contacts 656a and656b. This connects the stem voltage for the selected anode, nowappearing on leads 638 and 640, to the leads 201 and 202 of the grounddetector bus 418. The voltage is applied to the ground detector 408 inthe section box serving the multiplexer. As previously explained, theground detector is enabled by the ground detector enable bit on bus 4246so that its counters are reset and their input gates opened to receivethe two sequences of pulses that are derived from the stem voltage.

The Determine Status command includes an address only to reset aflip-flop 750 (FIG. 6a) for reasons that will later be explained.

To summarize, it takes three different commands to provide an indicationto the computer of the grounded or ungrounded status of an anode. Acommand Read Stem Voltage sets a function relay to connect the stemvoltage for the selected anode to leads 638 and 640. A command DetermineStatus conditions the ground detector 408 to conduct a 30 secondmeasurement, and connects the stem voltage on leads 638 and 640 to theground detector bus. Finally, after the measurement has been completed,a command Read Ground Detector reads out onto the data bus 416 from theground detector 408 a binary bit indicating whether the anode isgrounded or ungrounded. Read Cell Voltage. The purpose of this commandis to read the voltage drop between the anode bus 12 and the cathode bus23 (FIG. 1), and apply the voltage over data bus 416 and bus 420 to thedata processor. The command contains the address of the multiplexerassociated with the cell whose voltage is to be determined, a functioncode identifying the operation to be performed, and an enable bit.

The enable bit and the address reset the. function flipflops 616-625,and the function code with the address energizes the decoder 604 in themanner previously described. The decoder produces an output signal toset FF620. The output of FF620 energizes two solid state relays 658 and659 having contacts 658a and 659a associated therewith. When FF620 isset the contacts 658a and 659a close.

The contacts 658a and 659a are connected on one side to the leads 638and 640. On the other side, the contacts are connected by leads 42 and44 to the anode bus 12 and the cathode bus 23. When the contacts 658aand 659a close, the voltage drop across the cell appears on leads 638and 640. Ground detector bus FF625 is reset so the contacts 650a and650b are closed, as described above. Thus, the voltage drop across thecell is applied to the data bus 416 from whence it passes through thesection box to the data processor. Break Crust: During the reductionprocess it is necessary to break the crust which forms on the surface ofa cell so that more alumina may be added to the cell. In a typical cell,motor driven means may be provided to break the crust at one, the other,or both ends of the cell. Thus, there are three commands to controlcrust breaking, the commands differing only in their function codes.Each command includes the address, the function code, and an enable bit,which operate as described above to reset all of the function flipflops616-625, energize the decoder, and set one of the function flip-flops.Specifically, if the command is Break Crust End 1 FF623 (FIG. 6C) isset. If the command is Break Crust End 2 FF622 is set, and if thecommand is Break Crust Both Ends FF621 is set.

Referring now to FIG. 60, there is shown the logic power supply 670 forthe multiplexer, and an auxiliary power supply 672. Both power suppliesare connected to a source of power through a transformer 674. Thetransformer output leads 676 and 678 are connected through a pair ofnormally closed contacts 774e and 774f to a pair of leads 682 and 684. Aplurality of Triacs 686, 688 and 690 are connected tothe lead 682.

and are further connected in series with one of a plurality ofelectromechanical relays 692, 694, and 696. The relays are located in arelay panel remote from the multiplexer and each of the relays has apair of normally open contacts which may be connected in-parallel withmanual switches. The relay contacts or the manual switches may energizemotors to perform such functions as moving the anode bridge up or down,dumping of at one end or the other of a cell, or breaking the crust nearone, the other, or both ends of the cell.

The Triacs are controlled by the output of the function decoder 604. Ifthe command to be performed by the multiplexer is Break Crust End 1, thefunction signals will cause the decoder 604 to produce an output signalon lead 698 to set FF623. The output of FF623 energizes a solid staterelay 702. The relay 702 has a set of normally open contacts 702aconnected'between the gate electrode of Triac 688 and the lead 704. Thelead 704 is connected to one side of a set of contacts 706a on anauto-manual control switch 706 which is located remote from themultiplexer. The other side of switch contacts 706a is connected to thelead 684. If the switch 706 is in the auto position, indicating thatoperations are under data processor control, the closing of switchcontacts 702a causes Triac 688 to conduct thereby energizing relay 694.The relay contacts 694a close to energize a motor (not. shown) to breakthe crust at the designated end of the cell.

The function decoder 604 produces an output signal on a lead 707 to setFF 622 if the command to be performed is Break Crust End 2. FF622controls a solid state relay 704 having contacts 604a which in turncontrol Triac 690 to energize electro-mechanical relay 696 and close itscontacts 696a.

If the command to be performed is Break Crust Both Ends then thefunction decoder 604 produces an output signal on lead 710 to set FF621.This flip-flop has two solid state relays 714 and 716 connected to itsoutput. Relay 714 has a set of contacts 714a connected in parallel withthe contact 704a and the relay 716 has a set of normally open contacts716a connected in parallel with the contacts 702a. Thus, when FF621 isenergized both Triacs 688 and 690 are rendered conductive and bothrelays 694 and 696 are energized to close the contacts 694a and 696a.This energizes the motors (not shown) for driving the crust breakingapparatus at both ends of the cell.

Once begun, the crust breaking commands continue until the functionflip-flop 621, 622 or 623 is reset. The conditions for resetting thefunction flip-flops are discussed later. Move Bridge Up: When thecommand to be performed is that of moving the anode bridge up, thefunction decoder 604 produces an output signal on lead 718 to set FF624.The output from this flip-flop energizes a solid state relay 722 havinga set of normally open contacts 724a connected in the gating circuit ofTriac 686. When the Triac conducts it energizes electromechanical relay692 thereby closing the contacts 692a in a circuit which will supply thevoltage for driving a bridge jack motor. This voltage may be applied tothe bridge jack 19 (FIG. 1) over the leads 20 and 22 to move the anodebridge upwardly.

To avoid further repetition, the circuits responsive to the commands formoving the anode bridge down, or dumping alumina into one end or theother of the cell are not shown. Each of these circuits is energized byan output lead from the decoder 604 and includes a flipflop like 624, asolid state relay like 722, a Triac like 686, and an electro-mechanicalrelay like 692. Switch Status: This command is provided to enable thedata processor to determine whether the switch 706 is in the auto or themanual position. Output leads 730 and 732 of the auxiliary power supply672' are connected through switch contacts 706b and 7060 to a pair ofleads 734 and 736. These leads extend into FIG. 6b where they areconnected through normally open contacts 742a and 744a to the leads 638and 640. When the data processor wishes to determine the status ofswitch 706 associated with the multiplexer, it applies the Switch Statuscommand including an address, a function code, and an enable bit, to themultiplexer. The function flip-flops are reset as for commandspreviously described, and then the decoder 706 produces an output signalon leads 738 to set FF618. The output of this flip-flop energizes twosolid state relays 742 and 744 for closing the contacts 742a and 744a.If the switch 706 is set for Automatic control then switch contacts 706band 706c are closed and the output voltage of the auxiliary power supply672 is applied to the leads 638 and 640 through contacts 742a and 744a,from whence it passes through contacts 650a and 650b (closed becauseFF625 is reset) to the data bus 416 and eventually to the dataprocessor. On the other hand, if switch 706 is in the manual positioncontacts 706b and 706c are open and no voltage is applied across theleads 638 and 640 from the power supply. This condition is transmittedto the data processor over the data bus to signify that the switch is inthe manual position.

RESET AND ERROR CONTROL Each multiplexer is provided with circuits fordetecting various abnormal conditions resulting from cell upset, circuitfailures, or programming errors. Upon detecting any of these conditionsthe multiplexer generates an interrupt signal that is transmitted to thedata processor. Upon receipt of the interrupt signal the data processormay enter a diagnostic routine to discover what the abnormal conditionis and, depending upon the condition, possibly emit commands to themultiplexer to correct the condition.

The circuits for producing the interrupt signal are shown in FIG. 6a andinclude a flip-flop 750. This flipflop is connected to the output ofNAND gate 607 by a lead 751 so that FF750 changes state each time themultiplexer is addressed. That is, one addressing of the multiplexersets FF750 and the next addressing resets FF750. One output of FF750 isconnected to a NAND 752 and the other output is connected to a timercircuit 753 which has its output connected as a second input to NAND752.

Normally, FF750 is in a state such that one output blocks NAND 752. Whenthe multiplexer is addressed a first time, the address causes the outputof NAND 607 to go to a low level thus changing the state of FF750. Thesignal on lead 754 conditions one input of NAND 752 and the signal onlead 755 triggers the timer 753. After some predetermined interval, say45 seconds, the output of timer 753 conditions the second input of NAND752 if FF750 has not changed state as a result of a second addresscausing the output of NAND 607 to go low a second time.

Normally, the programming of the data processor should be such that themultiplexer is addressed a second time within 45 seconds after it isaddressed afirst time. The reason for this is that the first address maybe associated with a command such as Move Bridge Up.

As explained above, this causes circuits to energize a motor to move theanode bridge upwardly. If this command is not cancelled then the anodebridge might be moved upwardly to such an extent that one or more anodesmight be withdrawn from the electrolyte. Since the anodes carry currentsin the range of several tens of thousands of amperes, theopen-circuiting of the cell in this manner would obviously beundesirable.

Assuming that the second addressing of the multiplexer does not occurwithin 45 seconds of the first addressing, an interrupt signal isgenerated. After 45 seconds the output of timer 753 conditions NAND 752and, since FF750 has been triggered only once it further conditions NANDgate 752. The gate produces an output signal that is inverted at 756,inverted again by a NOR circuit 757, and applied to a single shotmultivibrator 758.

When multivibrator 758 receives a signal at its input, it triggers atimer 759 and applies a signal to a tri-state logic circuit whichincludes NOR circuits 760 and 761, inverter 762, and an AND gate763. Thetri-state logic circuit may be of a type such as the model DM8831 whichis commercially available from the National Semiconductor Co. Thetri-state logic circuit produces across the two leads in interruptcontrol bus 424f a voltage differential representing an interruptsignal. This signal exists until the timer 759 times out, and is appliedover bus 424f, through the section box, to the data processor.

The purpose of timer 759 is to prevent the tri-state logic circuit fromemitting several interrupt signals in a short interval of time as aresult of a single abnormal condition triggering multivibrator 758several times. This might occur as a result of pot voltae fluctuationswhich might trigger the multivibrator several times when an upsetcondition occurs in the cell, as described later. When the multivibrator758 is triggered it turns on timer 759. The output of timer 759, actingthrough NOR 760 maintains a high impedance at the output of thetri-state logic circuit'for a fixed interval of time after themultivibrator is first triggered. Thus, even though the multivibratormay be triggered by an output signal from NOR 756, then time out andreturn to its initial condition, and then in a very short interval betriggered again, the tri-state circuit will produce only one interruptsignal. At the time the multivibrator is triggered the second time, thetri-state logic circuit is being inhibited by the timer circuit 759 so asecond interrupt signal is not produced.

When an interrupt signal is generated because F F 750 is not resetwithin a given interval after it is set, the function flip-flops 616-625are reset and an alarm is sounded to call the operators attention to thecell. The

19 output signal from NAND 752 is inverted at 756 and applied over lead770 to NOR 630. The resulting output signal from NOR 630 is applied overlead 631 to FIGS. 6b and 60 where it resets the function flip-flops616-625.

The output of inverter 756 is also applied over a lead 771 to FIG. 60where it energizes a solid state relay 772. Relay 772 has a set ofnormally open contacts 772a connected in series with a set of normallyclosed contacts 773aand a solid state relay 774. The series circuit isdirectly connected across the output of transformer 674 so when contacts772a close, relay 774 is energized.

Relay 774 has a set of normally open contacts 774a connected in parallelwith contacts 7 72a. Contacts 774a close to provide a holding circuitfor holding relay 774a energized after relay 772 returns to thedeenergized state. An indicator lamp 775 is connected in parallel withrelay 774, and as long as the relay is energized the lamp is on tovisually indicate to an operator that an interrupt condition exists as aresult of failure to cancel a commanded function, or, stateddifferently, failure to reset F F750 within the required time.

Relay 774 controls normally closed contacts 774e and 774f so that whenthe relay is energized the contacts open. This removes power from thetriacs so that any function being controlled by the Triacs isimmediately terminated.

Relay 774 has a set of normally open contacs 7741; connected in serieswith manual switch contacts 706d and a speaker, bell, or other audiblealarm 776. The series circuit is connected across the output oftransformer 674 so when relay 774 is energized the alarm 776 is soundedif the switch 706 is in the auto position.

Relay 774 has two further sets of normally open contacts 774a and 774dconnected between the output leads 730 and 732 of the auxiliary powersupply 672 and two further leads 777 and 778.

To digress for a moment, a multiplexer may send an interrupt signal tothe data processor as a result of failure to reset FF750 within therequired time, as described above, or as a result of an over-voltageacross the cell, as subsequently described. Failure to reset FF750causes relay 774 to be energized as just described whereas theover-voltage condition causes an interrupt signal without energizingrelay 774. The course of action to be taken by the data processor isdetermined by what caused the interrupt, so means must be provided toenable the data processor to determine the cause. This is done by thedata processor by issuing a command called Determined Failure whichchecks the status of relay 774.

The Determine Failure command includes an address, a function code, andan enable bit. These signals function to reset function controlflip-flops 616-625 as previously described, and then enable decoder 604(FIG. 6b) to set FF619. This flip-flop controls two solid state relays779 and 780 having normally open contacts 779a and 780a. When FF619 isset, the contacts close thus connecting leads 777 and 778 to leads 638and 640. Since F F625 is reset, contacts 650a and 65012 are closed sothat the Determine Failure command places on data bus 416 the voltageappearing across leads 777 and 778. In FIG. 60, if relay 774 isenergized the contacts 7740 and 774d apply the output of the auxiliarypower supply 672 to the leads 777 and 778. On the other hand, if theinterrupt is the result of an overvoltage condition relay 774 will notbe energized so there will be no voltage across leads 777 and 778. Thus,the data processor will receive over the data bus either a voltagedifferential indicating the interrupt was caused by failure to resetFF750, or no voltage differential indicating the interrupt was caused byan overvoltage across the cell.

It should be noted that when the data processor receives an interruptsignal it cannot, from that signal alone, determine which of the cellson the pot line generated the interrupt. Thus, when an interrupt signalis received by the data processor it must generate one Determine Failurecommand for each cell on the pot line. These commands will differ fromeach other only in the address portion so that the multiplexers on thepot line are addressed in turn. By the response signal it receives.

over data bus 416, the data processor is able to identify which, if any,of the multiplexers generated an interrupt signal as a result of failureto reset a FF750. If, after addressing each multiplexer, the dataprocessor received no signal on data bus 416 as a result of a relay 774being energized, this is an indication that the interrupt was a resultof an over-voltage across a cell. The data processor may then beprogrammed to execute a sequence of Read Cell Voltage commands to locatethe cell which caused the interrupt.

If the data processor determines that the interrupt signal is a resultof the failure to reset a FF 750, it generates the command FailureReset. This command comprises a single binary bit on the bus 424d and isapplied to all the multiplexers for the pot line. In FIG. 6a, theFailure Reset command is applied to a differential receiver 781. Theoutput of the differential receiver is applied over a lead 782 to resetFF750. The output of the differential receiver is inverted by aninverter 783 and applied over a lead 784 to NOR 630. The output of NOR630 is applied to FIGS. 61b and 60 when it resets the functionflip-flops 616-625.

The output of inverter 783 is inverted by an inverter 785 and appliedover a lead 786 to FIG. 60 where it energizes a solid state relay 773.This relay controls normally closed contacts 773a so when the relay isenergized the contacts 773a open. This opens the circuit to relay 774and lamp 775. Relay 774 drops out so its contacts transfer. This shutsoff the audible alarm 766, disconnects the auxiliary power supply 672from the leads 777 and 778 and reapplies power to the Triacs. As soon asthe Failure Reset command is terminated, relay 773 returns to normal.The circuit is now cleared of its error-indicating condition.

As previously stated, the voltage drop across the cell is continuouslymonitoredand an interrupt signal produced if the voltage drop becomesexcessive. An abnormally high voltage drop generally indicates an upsetcondition in the cell that requires correction.

In FIG. 1, the voltage drop across the cell is available on leads 42 and44 connected to the anode bus 12 and the cathode bus 23. The lead 42(FIG. 6B) is connected to the parallel combination of a Zener diode 765and a solid state relay 766. Lead 44 is connected through a resistor 767and a Zener diode 768 to the other side of diode 765 and relay 7 66. Aslong as the voltage drop across the cell is within normal limits, say4.5V, diode 768 does not conduct and relay 766 is not energized.However, if the voltage drop across the cell should increase abovenormal limits for any of the reasons well known in the art, thebreakdown voltage of diode 768 will be exceeded and the diode willconduct, thus energizing relay 766. The relay closes'its contacts 766aso that a signal is applied over lead 790 to NOR 757 (FIG. 6a). Theoutput of NOR 757 triggers multivibrator 758 to generate an interruptsignal on bus424 as previously described. The diode 765 is connected inparallel with solid state relay 766 to protect the'relay by limiting thevoltage applied to the relay to the breakdown voltage of the diode.

While a preferred embodiment of the invention has been described inspecific detail, it should be understood that various modifications andsubstitutions may be made therein without departing from the spirit andscope of the invention as defined by the appende claims.

What is claimed is:

1. The method of detecting the position of an anode in an aluminareduction cell, said method comprising the steps of:

measuring the total current through said anode over an interval of timeand generating a reference value corresponding to the number of currentfluctuations which normally would occur during said time interval fromthe formation and release of gas bubbles at said anode when operatingthe cell with a properly adjusted anode for the current measured;deriving from said measured current a second value correspondingapproximately to the number of substantially sinusoidal currentfluctuations which actually occur during said interval of time; and,

comparing said reference value with said second value to produce asignal indicating maladjustment of the anode.

2. The method as claimed in claim 1 wherein:

said total current is measured by producing a voltage which varies assaid total current varies;

said voltage is converted to a sequence of pulses having a frequencyproportional to the rate of said current fluctuations; and,

said pulses are counted over said interval of time to produce saidreference value.

3. The method as claimed inclaim 2 wherein said voltage representingtotal anode current is filtered to eliminate components due to currentfluctuations at frequencies outside the range of frequencies selectedfor determining the rate at which gas bubbles are formed and released atthe anode, to thereby produce a second sequence of pulses, said pulsesof the second sequence being counted over said interval of time toderive said second value.

4. The method as claimed in claim 3 wherein said comparison stepcomprises comparing said reference value with said second value toproduce a first output signal when said reference value is greater thansaid second value or a second output signal when said reference value isless than said second value.

5. The method of detecting a grounded anode in a multiple anode aluminareduction cell, said method comprising:

sensing current flow through an individual anode to produce a voltageproportional thereto; converting said voltage to a sequence of pulsesrepresenting the rate at which anode current fluctuations would normallyoccur due to gas bubbles being formed and released at said anode whenoperating the cell with the anode properly adjusted;

counting said pulses over a predetermined interval of time to produce areference value; measuring the actual fluctuations in said voltage whichoccur within a selected range of frequencies 5 for indicating the rateat which gas bubbles are formed and released at said anode, to therebyproduce a second value; and v I comparing said reference value with saidsecond value to produce a signal indicating that said anode is groundedif said reference value is greater than said second value. 6. The methodas claimed in claim 5 wherein the measuring of said actual voltagefluctuations comprises .the steps of: v

filtering said voltage to remove therefrom fluctuations at frequenciesabove and below the range of frequencies selected for determiningtherate at which gas bubbles are formed and released at said anode tothereby producea second sequence of pulses; and, counting said pulses tothereby produce said second value. 7. The method as claimed in claim 6wherein the measuring of said actual voltage fluctuations furtherincludes:

rectifying said filtered voltage to obtain a rectified voltage;integrating said rectified voltage; comparing said integrated voltagewith said filtered voltage; and, producing a pulse in said secondsequence of pulses only when said filtered voltage exceeds apredetermined magnitude relative to said integrated volt- 8. Apparatusfor determining the position of an anode in an alumina reduction cell,said apparatus comprising:

means for producing a voltage representing current 40 flow through saidanode;

voltage to frequency converter means for converting said voltage to afirst sequence of digital pulses having a frequency proportional to therate at which current fluctuations would normally occur when said anodeis properly positioned;

digital counter means for counting said digital pulses to develop areference value;

pulse shaper means responsive to said voltage representing current flowfor producing a second se- 5O quence of pulses wherein each pulserepresents an actual fluctuation in anode current;

comparison means responsive to said counter means and said pulse shapermeans over a given interval of time for indicating maladjustment of theanode when said first sequence contains more pulses than said secondsequence during said interval. 9. Apparatus as claimed in claim 8wherein: said comparison means comprises a second counter meansresponsive to said second sequence of pulses and a digital comparatorfor comparing the counts in said counters.

10. Apparatus as claimed in claim 9 wherein said pulse shaper meanscomprises filter means responsive to said voltage for passing only thefluctuations in said voltage that corresponding to a range offrequencies selected for determining the rate at which gas bubbles areformed and released at said anode.

1. THE METHOD OF DETECTING THE POSITION OF AN ANODE IN AN ALUMINAREDUCTION CELL, SAID METHOD COMPRISING THE STEPS OF: MEASURING THE TOTALCURRENT THROUGH SAID ANODE OVER AN INTERVAL OF TIME AND GENERATING AREFERENCE VALUE CORRESPONDING TO THE NUMBER OF CURRENT FLUCTUATIONSWHICH NORMALLY WOULD OCCUR DURING SAID TIME INTERVAL FROM THE FORMATIONAND RELEASE OF GAS BUBBLES AT SAAID ANODE WHEN OPERATING THE CELL WITH APROPERLY ADJUSTED ANODE FOR THE CURRENT MEASURED; DERIVING FROM SAIDMEASURED CURRENT A SECOND VALUE CORRESPONDING APPROXIMATELY TO THENUMBER OF SUBSTANTIALLY SINUSODIAL CURRENT FLUCTUATIONS WHICH ACTUALLYOCCUR DURING SAID INTERVAL OF TIME; AND, COMPARING SAID REFERENCE VALUEWITH SAID SECOND VALUE TO PRODUCE A SINGLE INDICATING MALADJUSTMENT OFTHE ANODE.
 2. The method as claimed in claim 1 wherein: said totalcurrent is measured by producing a voltage which varies as said totalcurrent varies; said voltage is converted to a sequence of pulses havinga frequency proportional to the rate of said current fluctuations; and,said pulses are counted over said interval of time to produce saidreference value.
 3. The method as claimed in claim 2 wherein saidvoltage representing total anode current is filtered to eliminatecomponents due to current fluctuations at frequencies outside the rangeof frequencies selected for determining the rate at which gas bubblesare formed and released at the anode, to thereby produce a secondsequence of pulses, said pulses of the second sequence being countedover said interval of time to derive said second value.
 4. The method asclaimed in claim 3 wherein said comparison step comprises comparing saidreference value with said second value to produce a first output signalwhen said reference value is greater than said second value or a secondoutput signal when said reference value is less than said second value.5. The methoD of detecting a grounded anode in a multiple anode aluminareduction cell, said method comprising: sensing current flow through anindividual anode to produce a voltage proportional thereto; convertingsaid voltage to a sequence of pulses representing the rate at whichanode current fluctuations would normally occur due to gas bubbles beingformed and released at said anode when operating the cell with the anodeproperly adjusted; counting said pulses over a predetermined interval oftime to produce a reference value; measuring the actual fluctuations insaid voltage which occur within a selected range of frequencies forindicating the rate at which gas bubbles are formed and released at saidanode, to thereby produce a second value; and comparing said referencevalue with said second value to produce a signal indicating that saidanode is grounded if said reference value is greater than said secondvalue.
 6. The method as claimed in claim 5 wherein the measuring of saidactual voltage fluctuations comprises the steps of: filtering saidvoltage to remove therefrom fluctuations at frequencies above and belowthe range of frequencies selected for determining the rate at which gasbubbles are formed and released at said anode to thereby produce asecond sequence of pulses; and, counting said pulses to thereby producesaid second value.
 7. The method as claimed in claim 6 wherein themeasuring of said actual voltage fluctuations further includes:rectifying said filtered voltage to obtain a rectified voltage;integrating said rectified voltage; comparing said integrated voltagewith said filtered voltage; and, producing a pulse in said secondsequence of pulses only when said filtered voltage exceeds apredetermined magnitude relative to said integrated voltage. 8.Apparatus for determining the position of an anode in an aluminareduction cell, said apparatus comprising: means for producing a voltagerepresenting current flow through said anode; voltage to frequencyconverter means for converting said voltage to a first sequence ofdigital pulses having a frequency proportional to the rate at whichcurrent fluctuations would normally occur when said anode is properlypositioned; digital counter means for counting said digital pulses todevelop a reference value; pulse shaper means responsive to said voltagerepresenting current flow for producing a second sequence of pulseswherein each pulse represents an actual fluctuation in anode current;comparison means responsive to said counter means and said pulse shapermeans over a given interval of time for indicating maladjustment of theanode when said first sequence contains more pulses than said secondsequence during said interval.
 9. Apparatus as claimed in claim 8wherein: said comparison means comprises a second counter meansresponsive to said second sequence of pulses and a digital comparatorfor comparing the counts in said counters.
 10. Apparatus as claimed inclaim 9 wherein said pulse shaper means comprises filter meansresponsive to said voltage for passing only the fluctuations in saidvoltage that corresponding to a range of frequencies selected fordetermining the rate at which gas bubbles are formed and released atsaid anode.
 11. Apparatus as claimed in claim 10 wherein said pulseshaper means comprises: rectifier means responsive to said filteredvoltage to produce a rectified voltage; means for integrating saidrectified voltage to obtain the average value thereof; and, comparatormeans responsive to said integrating means and said filter means forproducing a digital output pulse when the output voltage of said filtermeans exceeds the output voltage of said integrating means. 12.Apparatus as claimed in claim 10 wherein: said filter means includesfirst and second filters connected in parallel circuits and bothresponsive to the voltage representing anode current flow; an inverterconnected to the output of said second filter; and, summing meansconnected to the outputs of said first filter and said inverter. 13.Apparatus as claimed in claim 12 wherein: said first filter comprisesmeans for filtering from said voltage fluctuations those frequenciesgreater than the highest frequency of said range; and, said secondfilter comprises means for filtering from said voltage frequenciesgreater than the lowest frequency of said range.
 14. The method ofdetermining a grounded anode condition in an alumina reduction cell,said method comprising the steps of: establishing a general relationshipbetween anode current and the period between fluctuations in anode stemvoltage for normal operation of the cell; measuring the anode currentand the period between its fluctuations for an anode whose condition isbeing determined; and determining if said measured period betweenfluctuations is greater than the corresponding period for saidestablished relationship, at said measured current, said measured periodbeing greater if said anode is grounded.